The VOR acts to counter head motion by causing rotation of the eyes in the opposite direction. There are separate circuits for rotational and translational movement of the head. The sensors for the former are the semicircular canals, and the sensors for the latter are the otoliths (the utricle and saccule). The circuits for the angular VOR are more straightforward (but still complex), and this section focuses on these pathways to illustrate how this reflex works; however, the basic scheme is the same: Vestibular afferent fibers go to vestibular nuclei, the vestibular nuclei in turn project to the various oculomotor nuclei, and motor neurons in the oculomotor nuclei give rise to axons that innervate the extraocular muscles. What varies are the specific vestibular and oculomotor nuclei that are involved.
With regard to the angular VOR pathways, the pathway for generating horizontal eye movement originates in the horizontal canals, and the analogous one for vertical movement originates in the anterior and posterior canals.
Fig.
9.27A shows the basic circuit for the horizontal VOR. Note that only the major central circuits originating in the left horizontal canal and vestibular nuclei are shown; however, mirror image pathways arise from the right canal and vestibular nuclei. Vestibular afferent fibers involved in the horizontal VOR pathway synapse primarily in the medial vestibular nucleus, which projects to the abducens nucleus bilaterally; inhibitory neurons project ipsilaterally, and excitatory ones project contralaterally. Control of the medial rectus muscle is achieved by abducens internuclear neurons that project from the abducens to the part of the oculomotor nucleus controlling the medial rectus muscle. Note the double decussation of this pathway results in aligning of the responses of functional synergists (e.g., the left medial rectus with the right lateral rectus).
The vertical VOR pathway involves primarily the superior vestibular nucleus, which has direct bilateral projections to the oculomotor nucleus.
Consider what happens in the horizontal canal pathway when there is head rotation to the left, as shown in
Fig.
9.27B.
Leftward head rotation would cause the visual image to slip to the right. However, compensation by the VOR would be triggered by depolarization of the hair cells of the left canal in response to the angular acceleration
CHAPTER 9 Organization of Motor Function •Fig. 9.27 Circuits Underlying the Horizontal Vestibuloocular Reflex (VOR). A, Thevestibularnucleireceiveexcitatoryinputfromtheafferentfibersofthehorizontalcanalandprojecttotheabducens(cranialnerveVI)nucleus.Thisnucleusinnervatesthelateralrectusmuscleandprojectstothecontralateraloculomotor(cranialnerveIII)nucleus,whichcontrolsthemedialrectusmuscle.Excitatoryneuronsareshowninred; inhibitoryones,inblue. Notethatonlythemajorpathwaysoriginatingintheleftvestibularnucleiareshown.Forclarity,onlythebeginningsofmirrorimagepathwaysfromtherightvestibularnucleiareshown(dotted lines). B, FlowofactivityintheVORcircuitryinducedbyleftwardheadrotation.Increasedaxonalthicknessindicatesincreasedactivity;thinneraxonsindicatedecreasedactivityincomparisonwithlevelsatrest(A). Notethatleftwardrotationcausesbothanincreaseinactivityoftheleftvestibularafferentfibersandadecreaseinactivityoftherightones.MLF,mediallongitudinalfasciculus;vestibularnuclei:I,inferior;L,lateral;M,medial;S,superior.